Immunoglobulin G (IgG) Receptors (Fc.gamma.R)
An IgG antibody molecule has two binding determinants (known as Fab) for its specific cognate antigen, and one binding determinant (Fc) for its cellular receptor (Fc.gamma.R; reviewed in Ravetch, J. V. and J-P. Kinet 1991, Ann. Rev. Imm. 9:457). There are three classes of Fc.gamma.R, designated Fc.gamma.RI, Fc.gamma.RII and Fc.gamma.RIII. Fc.gamma.RI has the highest affinity for IgG Fc, and is present on monocytes and macrophages, and on human neutrophils when induced by gamma-interferon (IFN-.gamma.), which also enhances Fc.gamma.RI expression on monocytes and macrophages (Guyre, P. M. et al., 1983, J. Clin. Investig. 72:393). Interleukin-10 (IL-10) is also a potent up-regulator of Fc.gamma.RI expression on monocytes (Capsoni, F. et al., 1995, J. Leukoc. Biol. 58:351). Neutrophil expression of Fc.gamma.RI is increased almost four-fold by granulocyte-colony stimulating factor (G-CSF; Repp, R. et al., 1991, Blood 78:885), and about three-fold during streptococcal pharyngitis (Guyre, P. M. et al., 1990, J. Clin. Investig. 86:1892). G-CSF stimulation is associated with enhanced neutrophil-mediated cytotoxicity of tumor cells (Gosselin, E. et al., 1992, J. Immunol. 149:3477).
Cellular distribution of the low affinity receptor Fc.gamma.RII is broader, extending to neutrophils, platelets, B cells, T cell, and possibly mast cells and basophils, in addition to monocytes and macrophages. Human Fc.gamma.RIII receptors are restricted to macrophages and macrophage cell lines, natural killer (NK) cells, myeloid precursor cells, and a neutrophil cell line (Ravetch and Kinet, supra).
The interaction of antibody-antigen complexes with cells via Fc.gamma.R triggers a range of responses including antibody-dependent cytotoxicity, degranulation, phagocytosis, and immunomodulatory signals that regulate lymphocyte proliferation and antibody secretion. Fc.gamma.RI, for example, is active in enhancing antigen presentation (Valerius, T. et al., 1993, Blood 82:931) and the Fc.gamma.RI complementarity designation CD64 has been used to distinguish granulomonocytic progenitor cell populations within a human progenitor cell mixture (CD34.sup.+ /CD38.sup.+ ; European Patent Application EP O 708 336 A2).
Further, a monoclonal antibody (mAb) that binds Fc.gamma.RI has been shown to cause down-regulation of expression of this receptor, and administration of this mAb to a chronic immune thrombocytopenia purpura patient resulted in clinical improvement (Ericson, S. G., 1996, Brit. J. Haematol. 92:718). The analysis of in vivo function of Fc.gamma.RI receptors is complicated by the finding that four healthy members of one family lack Fc.gamma.RI (Ceuppens, J. L., 1988, J. Clin. Invest. 82:571). (Cell attaining with PE-Cy5
The tandem dye PE-Cy5 comprises the combination of two fluorescent dyes, R-phycoerythrin (PE) and Cy5.18.OSu (Cy5; Mujumdar, R. B. et al., 1993, Bioconj. Chem. 4:105). PE-Cy5 is commonly used conjugated to a monoclonal antibody (mAb), as described in for example, the Jan. 1, 1996, issue of Blood that reviews the structure and biology of CD34, accompanied by a cover photograph of a flow cytometric experiment with PE-Cy5 conjugated to an anti-CD34 mAb (Krause D S, et al., 1996, Blood 87:1). PE-Cy5 tandem dye is known as "tricolor" since it is frequently used with other dye-mAb conjugates for three-color flow cytometric analyses (Waggoner A S, et al., In: Clinical Flow Cytometry, p.185 (Eds) A. Landay et al. The New York Academy of Sciences, New York, N.Y., 1993).
A non-toxic molecule that binds specifically to Fc.gamma.RI in vivo would be useful in a variety of diagnostic and therapeutic applications.